1. Field of the Invention
The present invention relates to a process for producing light olefins from hydrocarbon feedstock, and more particularly to a process for producing light olefins at high yield with high selectivity from hydrocarbon feedstock using a catalyst which, even in an atmosphere of high temperature and humidity, has a relatively stable structure, thereby maintaining its catalytic activity over a long period of time, and shows hydrothermal stability.
2. Description of the Related Art
Olefins, particularly light olefins, such as ethylene and propylene, are widely used in the petroleum chemical industry.
These light olefins are generally produced by the thermal cracking (steam cracking) of naphtha in the presence of steam. The steam cracking technology is being improved in many fields in order to cope with high process temperature and a reduction in residence time and to optimize energy efficiency. However, it is not easy to improve energy efficiency merely by simple improvements in engineering technology, and the steam cracking process currently accounts for about 40% of the total energy required in the petroleum chemical industry. Accordingly, to reduce environmental pollution and increase economic efficiency, there is a need for improved process technologies for the optimization of energy, the reduction of feedstock use, the minimization of carbon dioxide discharge, etc. Also, light naphtha is typically used as feedstock, but is expensive compared to full-range naphtha as described later, and thus, will necessarily act as a limitation in increasing economic efficiency. Particularly, in the steam cracking technology, that is currently applied, not only it is not easy to control the composition of olefins but also the reaction temperature is a level of 800-900° C., indicating a requirement for a large amount of thermal energy. Thus, a need for improvement in steam cracking technology has suggested.
Also, light olefin compounds can be produced by a fluid catalytic cracking (FCC) process. This FCC process is widely known in the art as catalytic cracking technology using a catalyst having the form of fine particles, which behaves like fluid when treated with steam. Particularly, deep catalytic cracking (DCC) technology is known which is a process developed by modifying the FCC process in order to increase the yield of olefins (mainly, propylene) other than gasoline. In the FCC process, a heavier fraction than full-range naphtha used in the present invention, such as vacuum residue, atmospheric residue, or gaseous oil, is used as feedstock.
Regarding the production of olefins, in addition to the above-described steam cracking and FCC processes, olefin conversion processes using catalytic cracking have been proposed. In most of these processes, the HZSM-5 catalyst as a solid acid catalyst is widely used. However, in the conventional catalytic cracking processes using the solid acid catalyst, the reaction temperature is typically at least 650° C., and at least 30% of the reaction feed is steam. The porous solid acid catalyst (e.g., zeolite) used in these catalytic cracking processes has problems in that, when it is placed in a steam atmosphere of more than 500° C., the dealumination of its tetrahedral framework will occur to cause structural breakdown thereof and at the same time, the acid sites of the solid acid catalyst will be reduced, resulting in a rapid reduction in catalytic activity and reactivity.
Accordingly, in the above-described conventional light olefin production processes including the catalytic cracking process, studies are actively performed to decrease the instability of the catalyst, and thus, a reduction in process performance, which occur when the catalyst is placed in a severe process atmosphere of high temperature and humidity.
Regarding these studies, U.S. Pat. No. 6,867,341 discloses a naphtha cracking catalyst obtained by controlling the distribution of aluminum atoms and crystal size of zeolite, as well as a process for cracking naphtha using this catalyst. According to the disclosure of said patent, the catalyst is designed so that the production of aromatic compounds on the pore surface can be minimized by chemically neutralizing aluminum present outside the pores, whereas ethylene and propylene, having small sizes, can be more selectively produced by increasing the concentration of aluminum ions inside the pores to increase the number of acid sites. Meanwhile, as disclosed in said patent, when a ferrierite zeolite catalyst obtained by this technology is used in catalytic cracking, the reactivity of the catalyst will become excellent even in a relatively severe process environment, such as maintaining the catalyst in an atmosphere of 50% steam at 690° C. for 2 hours. Regarding the hydrothermal stability of the catalyst, however, it is expected that the structural stability and reactivity of the catalyst cannot be secured when it is treated with 100% steam at 750° C. for 24 hours.
U.S. Pat. No. 6,835,863 discloses a process for producing light olefins by catalytically cracking naphtha (boiling point: 27-221° C.) using a pelletized catalyst containing 5-75% by weight of ZSM-5 and/or ZSM-11, 25-95% by weight of silica or kaolin and 0.5-10% by weight of phosphorus. However, there is no mention of hydrothermal stability in a severe environment of high temperature and humidity.
Japanese patent laid-open publication No. Hei 6-192135 discloses a catalytic cracking process for producing ethylene and propylene from C2-12 paraffin-containing light naphtha (density: 0.683 g/cc; composition: 42.7 wt % n-paraffin, 36.1 wt % iso-paraffin, 0.1 wt % olefins, 14.0 wt % naphthene, and 7.1 wt % aromatics; and the distribution of the paraffin component: 0.1 wt % C3, 5.2 wt % C4, 18.7 wt % C5, 19.0 wt % C6, 15.2 wt % C7, 13.5 wt % C8, 6.1 wt % C9, 0.1 wt % C10 and 0.1 wt % C11) using HZSM-5 and HZSM-11 catalysts (molar ratio of SiO2/Al2O3: 150-300) at a temperature of 620-750° C. and a WHSV of 1-200 h−1. According to the disclosure of said patent, under reaction conditions of 680° C. and a WHSV of 25 h−1, a conversion rate of 93.6 wt % and ethylene+propylene production of 44.9 wt % are shown. However, the HZSM-5 or HZSM-11 catalyst is used in the catalytic cracking reaction in an unpelletized state, and steam or inert gas is not fed during the reaction. Thus, the catalyst has excellent initial activity, but there is a possibility for the catalyst to be easily inactivated. For this reason, it is expected that the reactivity of the catalyst in a severe environment of high temperature and humidity will be remarkably reduced.
Meanwhile, Japanese patent laid-open publication No. 6-199707 reports that, in a catalytic cracking process for producing ethylene and propylene as main products from light naphtha containing C2-12 paraffin, the use of a proton-zeolite (SiO2/Al2O3=20-500) catalyst loaded with 100 ppm iron (Fe) allows light olefins to be produced with good selectivity. The catalyst has excellent initial activity since steam or inert gas is not fed during the reaction, but there is a possibility for the catalyst to be easily deactivated in a high-temperature reaction involving steam. For this reason, it is expected that the reactivity of the catalyst in a severe environment of high temperature and humidity will be remarkably reduced.
Accordingly, there is an urgent need for the development of a process where reaction activity is maintained even in a severe process environment of high temperature and humidity so that light olefins, such as ethylene and propylene, can be selectively produced with high conversion and selectivity from reaction feedstock, particularly full-range naphtha.